Process for the rapid tanning of leather



United States Patent US. Cl. 8-94.17 8 Claims ABSTRACT OF THE DISCLOSUREAn improvement in the acidification step of rapid tanning processes fordelimed or non-delimed pelts, which comprises the steps of 1)acidification, (2) chrome pretanning, and (3) tanning, by conducting theacidification with mixtures of (l) mineral acid or acidic mineral acidsalts and (2) salts of naphthalene sulfonic acids. Preferably, theacidification mixture contains a mineral acid and ammonium salts ofnaphthalene sulfonic acids or alkali metal salts of naphthalene sulfonicacids and an ammonium salt. Salts of sulfurous acid and neutralinorganic salts can also be added. The acidification mixture is of aconcentration to produce pelts having a :pH below about 6.5. Thisprocess can be carried out in the same drum with all three steps in theabsence of liquor.

DISCLOSURE A number of rapid tanning processes for the production ofvegetable-tanned leather are known, in which the pelts are pretreatedwith acids in the form of a pickle before the final tanning withvegetable tannins. If mineral acids or lower fatty acids are used forthis purpose, the addition of neutral salts, e.g. sodium chloride, isnecessary to prevent an undesired swelling of the pelts. Such apretreatment by pickling is customary, especially in those cases wherethe pelts are pre-tanned with chromium tanning agent before the finaltanning.

It has already been proposed to use organic acids which have no swellingaction, particularly naphthalenesulphonic acids, for acidifying thepelts before the tanning with vegetable tannins. When these acids areused, the addition of salts is not necessary and there is also achievedan accelerated penetration of tannin with the vegetable tannins and abrightening of the leather.

To acidify the pelts, a specific amount of acid is required which iscomparatively high for organic acids without swelling action, due totheir high equivalent weight. In comparison with mineral acids, such assulphuric acid, these organic acids are moreover only obtained at arelatively high price, and their practical use is therefore limited foreconomic reasons. When naphthalene-sulphonic acids are used asacidifying agent, it must also be taken into account that these acidswhich have, moreover, a strong corrosive effect on metals, are highlyhygroscopic and are only available in the form of pastes or blocks whichare difficult to handle. It is therefore expedient to dissolve theseacids in water before application. Consequently, they are not verysuitable to be used for the acidification of pelts in the drum without aliquor.

A rapid tanning process has now been found in which the pelts areacidified with mixtures of mineral acids or salts having a mineral acidreaction in aqueous solution, and of salts of naphthalene-sulphonicacids, pre-tanned with chromium tanning agent, and then finally tannedwith vegetable tannin and/ or syntans. Compared with the use of freenaphthalene-sulphonic acids for acidification, the mixtures which areconsiderably easier to handle lead to an at least equally rapid tanninpenetration in the final tanning process with vegetable tannins. It hasalso been found that this new process which is the object of the presentapplication, can be carried out with particular advantage without aliquor.

According to the new process, the pelts which may be delimed and wettedin the usual manner are acidified with mineral acids or salts having amineral acid reaction in aqueous solution and with salts ofnaphthalenesulphonic acids, possibly in the presence of water-solubleinorganic neutral salts or carboxylic acids. The acids are used in suchan amount that at the end of the treatment the pelts exhibit a pH valuebelow 6.5, preferably of 3.0- 3.6, throughout their cross-section. Theacidified pelts are then pre-tanned in known manner with (1)chromiumtanning agents or (2) chromium complex tanning agents or (3)mixtures of chromium tanning agents and (a) syntans and/or (b)naphthalene-sulphonic acids and/or (0) condensation products ofnaphthalene-sulphonic acids and formaldehyde and/or (d) sulphite wasteliquor, and finally tanned with vegetable tannins and/or syntans. Thechromium tanning agents (1)-(3) can be used by themselves or asmixtures.

When non-delimed pelts are used as starting material, the alkaline peltsare acidified with mineral acids and salts of naphthalene-sulphonicacid, the ammonium or alkali metal salts being preferably used as saltsof naphthalene-sulphonic acid. If alkali metal salts of naphotherWater-soluble ammonium salts is advantageous in the case where alkalinepelts, i.e. non-delimed and nonwetted pelts, are employed as startingmaterial. Furthermore, it is advantageous that salts of sulphurous acidor carboxylic acids, and optionally other water-soluble inorganicneutral salts, besides the ammonium salts, are present.

As salts of naphthalene-sulphonic acids there are preferably used thealkali metal or ammonium salts of fi-naphthalene-sulphonic acid or oftechnical mixtures substantially containing 18-naphthalene-sulphonicacid. Salts of anaphthalene-sulphonic acid, 1,5- or1,6-naphthalene-disulphonic acid or 1,3,6- or1,3,7-naphthalene-trisulphonic acid are also suitable. Furthermore,neutralised technical mixtures which are obtained in the production ofnaphthalenedisulphonic acids as a by-product and mainly consist ofsulphuric acid and naphthalene-disulphonic acids can be used withparticular advantage. The amounts of such salts of naphthalene-sulphonicacids employed, in general, lie at about 0.5-10%, preferably 1-6%,referred to the pelt weight.

As mineral acids, sulphuric acids, hydrochloric acid or phosphoric acidare especially suitable. It is also possible to use salts having amineral acid reaction in aqueous solution, such as sodium or potassiumbisulphate. The amount of mineral acids to be used depends on the pHvalue which the pelts are to have at the end of the treatment.

Examples of water-soluble inorganic neutral salts are, interalia, sodiumsulphate, sodium chloride, potassium sulphate, potassium chloride,ammonium sulphate, ammonium chloride, and other known water-solubleinorganic salts of neutral reaction. These salts are generally used inamounts of about 05-10%, referred to the pelt weight.

Carboxylic acids which are especially suitable for being used for thepresent process are those of the aliphatic series, such as formic acid,acetic acid, propionic acid, adipic acid, glutaric acid, oxalic acid andhydroxycarboxylic acids, such as lactic acid, as well as citric acid. Asexamples of the aromatic series, there may be mentioned: sulphophthalicacid and sulphosalicyclic acid. Mixtures of carboxylic acids can also beused. The carboxylic acids are normally used in quantities of 0.52%formic acid or of an equivalent amount of another carboxylic acid,referred to the pelt weight.

Ammonium salts of acids other than naphthalene-sulphonic acids, whichcan be concurrently used are, inter alia, water-soluble ammonium saltsof inorganic acids, such as ammonium sulphate and ammonium chloride, orwater-soluble ammonium salts of organic acids, such as ammonium acetate,ammonium formate and ammonium phthalate. Mixtures of such ammonium saltsare also frequently employed with advantage.

By the concurrent use of ammonium salts and carboxylic acids there isachieved a more uniform and faster acidification of the pelts. Theaddition of salts of sulphurous acid removes the hydrogen sulphideliberated in the acidification of the alkaline pelts.

It was not to be foreseen that it would be possible to achieve with themixtures of mineral acids and salts of naphthalene-sulphonic acidsaccording to the invention and at least equally satisfactoryacceleration of the tanning agent penetration in the final tanning withvegetable tannins, as can be obtained with free naphthalene-sulphonicacid. It has hitherto generally been assumed that only the free acidswhich have no swelling action have these advantageous effects when usedas salt-free pickle. Compared with the free naphthalene-sulphonic acids,the salts thereof have the advantage of being considerably easier tohandle. Any detrimental effect on the final tanning with vegetabletannins, caused by the neutral salts introduced during acidification ofthe pelts with the mixtures according to the invention can be obviatedby rinsing the pelts after pre-tanning with chromium tanning agents,without the specific effect of the acidification according to theinvention being lost.

According to an advantageous method of carrying out the new process, thepelts which may be delimed and wetted are acidified with mixtures ofmineral acids and salts of naphthalene-sulphonic acids, possibly in thepresence of water-soluble inorganic neutral salts and carboxylic acids,without a liquor, then pre-tanned in the usual manner with chromiumtanning agents, and finally tanned with vegetable tannins and/orsyntans.

Suitable agents for the acidification of pelts without a liquor are, inparticular, powdered mixtures of salts of naphthalene-sulphonic acidswith sodium and/or potassium bisulphate. It is equally advantageous touse spray-dried alkali metal or ammonium salts of technical mixturescontaining naphthalene-disulphonic acids and possiblynaphthalene-trisulphonic acids, such as are obtained as by-product inthe production of naphthalenedisulphonic acids, in admixture withbisulphates, as powdered agents for the acidification of pelts.

The pre-tanning with chromium tanning agents or chromium complex tanningagents or mixtures of chromium tanning agents and the components (a) to(d) mentioned above is carried out by known methods. AS a rule, amountsof chromium tanning agents are employed which would be insufficient forcomplete tanning. Pre-tanning according to the process described in US.Patent No. 2,997,857 is especially suitable. Pre-tanning without aliquor can be carried out with particular advantage according to US.Patent 3,254,937.

In this process 100 kg. of unsplit cow butt is treated with a spraydried mixture of 225 pts. of Waste sulfite liquor, 300 pts. H 0, 300pts. basic chromium sulfate and 150-320 pts. of a condensate from thereaction by formaldehyde with phenol with or without crude cresols withsolid NaOH as a catalyst and with S blown in after condensation.

A dry mixture of chrome tanning agent with naphthalene SO Na or 33%basic chrome tanning salt with .3% formic acid can also be used topretan.

Suitable chromium pre-tanning agents are chromic sulphates ready fortanning which are directly placed into the tanning drum in powderedform. The powdered chromium tanning agents which are the object of [1.5.

Patent No. 3,174,817 wherein hides are chrome tanned with powdered driedchromic sulfate or basic chrome sulfate, which has been masked withacetic acid are particularly suitable. Chromium complex tanning agentsobtained by mixing chromic salts with syntans prepared bysulphomethylation of phenol/formaldehyde condensation products areespecially useful. Data regarding the synthesis of such tanning agentscan be found in US. Patent No. 2,997,857.

Mixtures of chromic salts with naphthalene-sulphonic acids or withcondensation products of naphthalenesulphonic acids and formaldehyde,especially with sodium salts of such condensation products, or mixtureswith sulphite Waste liquor, especially with sodium salts of delimedsulphite waste liquors, are also very advantageous.

The pre-tanning agents (1) to (3) are preferably used in such amountsthat the finished leather has a chromium oxide content of less than 1.5%Cr O preferably less than CI'203.

Final tanning is carried out with vegetable tannins and/or syntans inknown manner.

It has proved to be particularly advantageous that according to the newprocess the final tanning can be performed without a liquor. Even whenworking on an industrial scale, the risk of case-hardening or of anundesirable strong formation of drawn grain does not occur.

A preferred method of carrying out the new process consists in that theacidification of the pelts as well as the pre-tanning with chromiumtanning agents or chromium-containing tanning agents and also the finaltanning wiih vegetable tannins and/or syntans are carried out without aliquor. This offers the advantageous possibility of continuouslyprocessing the material from the alkaline pelts to the finished leatherin a single operation in one and the same drum. It is expedient to rinsethe pelts before the final tanning in order to remove the saltsintroduced by the alkaline pelts and, after discharging the rinsingliquors, to carry out the final tanning with vegetable tannins and/orsyntans in the same drum without a liquor.

Compared with the process of US. Patent 3,254,937, the new processprovides a simple method of operation, a faster tanning agentpenetration and, particularly when carried out on an industrial scale,leather with noticeably less drawn grain or with none at all.

In the preferred method of carrying out the new process, in which theacidification as well as the pretanning and the final tanning areperformed without a liquor, it has proved advantageous to initiate thefinal tanning with powdered syntans. As described in the process of US.Patent No. 3,253,879, this patent shows a chrome tanning salt or basicchrome tanning agent mixed with a syntan produced by condensing aphenol, an aldehyde and a sulfite or a bisulfate. This chromed syntan isapplied to a delimed hide. It is possible to use for this purpose thosesyntans which have a good dispersing action, are readily soluble, andhave themselves a good tanning action. In particular US. Patent 3,253,-879 describes a process for the rapid tanning of medium and heavyleathers consisting essentially of (A) pretanning delimed hides with anormal or basic tanning chromium salt and a syntan obtained by thecondensation of a phenol with an aliphatic aldehyde and with a memberselected from the group consisting of a sulfite and a bisulfite;

(B) drumming the pretanned hides before final tanning with a powderedwell dispersing and readily soluble exchange tanning agent; and

(C) finally tanning the hides by drumming with an effective amount of apowdered vegetable tanning extract only.

Other syntans of this type are e.g. exchange syntans such as aredescribed in US. Patent 1,901,536 and are made by condensingnaphthalenes or tetrahydronaphthalenes sulfonated at 140-160" C., andcondensing the sulfonated naphthalene with formaldehyde and dihydroxydiphenylsulfones. Sulfur chloride can also be used in the condensation.Even more suitable for initiating the final tanning without a liquor areauxiliary syntans and/or sulphite waste liquor, possibly used togetherwith up to 50% of exchange syntans, referred to the amount of pretanningagent used. The final tanning without a liquor is advantageously carriedout with those condensation products of naphthalene-sulphonic acids andformaldehyde and/ or with those sulphite waste liquors which areadjusted to an acid number of at least 30, preferably 50 to 120, withinorganic or organic acids and which may also contain 520% of theirweight of water-soluble inorganic neutral salts.

The term acidification or tanning without a liquor in the meaning of thepresent invention comprises those processes in which the agents foracidification or tanning are caused to act on the pelt material or theleather in the wet state but without water or substantial amounts ofwater. Any alkaline swollen pelt yields water in the course of theacidification. As a rule, this amounts to about -25%, referred to thepelt weight, that is to say that in the case of the acidificationwithout a liquor as preferred according to the present invention, acorresponding amount of liquor accumulates in the course of theoperation. This liquor can be discharged before the pre-tanning withchromium tanning agents or chro-' mium-containing tanning agents. Withinthe scope of the present invention, however, pre-tanning in the presenceof this liquor separated during acidification is also possible withoutdisadvantage. When operating without a liquor, the products are normallyused in powdered form, but the use of solid products, e.g. in blockform, or of highly concentrated extracts, is also possible.Acidification and tanning without a liquor are mainly performed bymilling the pelt or leather with the acidifying agents or tanning agentsin a rotating drum.

The new process is particularly suitable for the production of splithides, welting leather, insole leather, sole leather, sole leathersplits and water-proof leather.

In the following Examples which are given for the purpose ofillustrating the invention, the parts by weight stand for kilograms. Thepercentages are referred to the weight of the pelt.

Example 1 Split hide pelts (splitting substance 2 mm.) which have beendelimed and bated in the usual manner are pickled with 60% water at C.,0.8% sulphuric acid (66 B.), 1.5% calcium formate and 4% sodium 5-naphthalene-sulphonate. After a drumming time of about 2 hours, thepelts are evenly acidified. Pre-tanning is then carried out in thepickling liquor with 2.5% of a powdered 33% basic chromium sulphate(content of chromium oxide After a drumming time of about 2 hours, thepelts are evenly tanned. The tanning liquor is drained off and thepre-tanned pelts are rinsed.

The pre-tanned leather is finally tanned in the drum with a vegetabletanning liquor of 5 B. The tanning liquor employed consists of parts byweight pure tannin of mimosa, parts by weight pure tannin of a sulphitedquebracho extract and 15 parts by weight pure tanning agents of asoft-tanning synthetic exchange tanning agent as is obtainable from 1mol 4,4-dihydroxydiphenyl-sulphone, 0.85 mol ,B-naphthalene-sulphonicacid and 1 mol formaldehyde.

The leather is completely tanned after a tanning time of about 2 days. Asmooth leather with a gentle feel is obtained after the usual finishing.

The pre-tanned leather can also be finally tanned in a series ofsuspenders with 5 pits of 2-6 B. In this case a tannin mixture ofsulphited quebracho and mimosa is used in a ratio of equal partsbyweight of pure tanmin. The leather is completely tanned after a tanningtime of about 1 week and finished in the usual manner. A smooth andplump leather is obtained.

Example 2 Split hide pelts which are suitable for the production ofsplit hides are delimed and bated in the usual manner. The pelts arethen drummed without a liquor with 5% of a powdered mixture of 50 partsby weight sodium or potassium bisulphate, 25 parts by weight ammoniumsulphate or ammonium chloride and 25 parts by weight sodiumB-naphthalene-sulphonate with the concurrent use of 0.9% of 85% formicacid (diluted with water 1:1). After a treatment time of about 1 /2 to 2hours, the pelts have a pH value of about 3.2 to 3.4 throughout theircross-section.

The liquor formed by the elimination of water from the pelts duringacidification is left in the drum and 4% of a powdered chromium complextanning agent as described in Example 1 of U.S. Patent 3,254,937 areadded. The acidified pelts are drummed without the addition of Water,until the whole cross-section of the pelts is tanned through. This isnormally achieved after a tanning time of 1 /2 to 2 hours. Thepre-tanned pelts are subsequently rinsed.

After discharging the rinsing liquor, the pre-tanned pelts are furtherdrummed in the same drum with 8% of a powdered precursory tanning agent.The precursory tanning agent consists of parts by Weight of a spraydriedcondensation product which is obtained from a technicalnaphthalene-sulphonation mixture chiefly containingfl-naphthalene-sulphonic acid, and present in the form of the ammoniumsalt, and 0.65 mol formaldehyde per mol naphthalene-sulphonic acid, 10parts by weight sodium sulphate and 10 parts by wegiht adipic acid.After a drumming time of 2-3 hours, the pelts are evenly tanned through.

For final tanning the pelts are first drummed in the same drum with 10%pure tannin of a powdered sulphited quebracho extract without a liquorfor 1 hour. Then there are added 4% pure tannin of a powdered sweetenedchestnut wood extract, 2% of the same powdered quebracho extract and 2%of a commercial soft-tanning syn thetic exchange tanning agent, as isobtainable from 1 mol 4,4-dihydroxy-diphenyl-sulphone, 0.9 molfi-naphthalene-sulphonic acid and 1 mol formaldehyde.

After a total tanning time of about 4-5 hours, the leather is completelytanned and is greased and finished in the usual manner. A light-colouredleather of level shade is obtained, which exhibits only a negligibledrawn grain or none at all.

, (splitting substance 3 mm.) prepared in the usual manner in the limehouse, are treated in the drum in one operation, from the alkaline limedpelt to the completely tanned leather, without previous scudding, with5% of the powdered mixture described below, 0.2% sodium metabisulphite,0.8% of formic acid diluted with water 1:1 and 0.2% acetic or lacticacid. No liquor is added. After 30 minutes to one hour, the pelts areevenly acidified and have a pH value of about 3.2 to 3.4 in thecrosssection. The small amount of liquor formed (about 15 to 20%) isdischarged.

The powdered mixture used, for acidification of the pelts consists of 50parts by weight sodium bisulphate and 50 parts by weight of a technicalmixture neutralised with ammonia and spray-dried and containing about65% sulphuric acid, about 35%, 1,5- and 1,6-naphthalenedisulphonic acid,and small amounts of 1,3,6- and 1,3,7- naphthalene-trisulphonic acid.

The acidified pelts are subsequently further drummed in the same drumwith 4% powdered pre-tanning agent.

The pre-tanning agent consists of 50 parts by weight of a 33% basicchromium sulphate (chromium oxide content 25 and 50 parts by weight of acondensation product present in the form of the sodium salt and obtainedfrom B-naphthalene-sulphonic acid and formaldehyde (molar ratio 1:0.65mol).

After a drumming time of 2 hours, the pelts are evenly tanned through.The leather is subsequently Washed for 10 minutes and rinsed at 30 C.for 5 minutes. The rinsing liquor is completely discharged.

The leather is then further drummed without a liquor with 6% of apowdered synthetic precursory tanning agent.

The precursory tanning agent consists of 70 parts by weight of aspray-dried condensation product present in the form of the ammoniumsalt and obtained from a technical naphthalene-sulphonation mixturemainly consisting of ,B-naphthalene-sulphonic acid and formaldehyde in amolar ratio of 1 mol naphthalene-sulphonic acid to 0.9 mol formaldehyde,which has been adjusted to an acid number of 120 by the addition ofglutaric acid and also contains 30 parts by weight of a purified anddelimed sulphite waste liquor.

After a drumming time of 2 hours, the leather is evenly tanned through.Final tanning is subsequently carried out in the same drum by theaddition of 7% of a powdered sulphited quebracho extract and 7% of apowdered mimosa extract. After a treatment of 34 hours the leather iscompletely tanned, then greased and finished in the usual manner. Asmooth leather is obtained which exhibits only a negligible drawn grainor none at all.

Example 4 500 parts of alkaline cow butt pelts prepared in the beamhouse by the methods customary for the production of sole leather arelevelled (thickness 4.2 mm.) and treated in the drum, without theaddition of water, with 5% of the powdered mixture described below, 0.2%sodium metabisulphite and l.21.5% of 85% formic acid for about 3-4hours. The liquor emerging from the alkaline swollen pelts has a pHvalue of about 3.2-3.5. The pelts are evenly acidified throughout theircrosssection.

The powdered mixture consists of 50 parts by weight sodium bisulphate,25 parts by weight ammonium sulphate and 25 parts by weight of atechnical naphthalenesulphonation mixture present in the form of theammonium salt and mainly consisting of fi-naphthalenesulphonic acid.

After discharging the separated liquor, pre-tanning is carried out with4% of a powdered chromium-containing I synthetic tanning agent.

The pre-tanning agent consists of 60 parts by weight chromic sulphateand 40 parts by weight of a condensation product present in the form ofthe sodium salt and obtained from fl-naphthalene-sulphonic acid andformaldehyde in a molar ratio of 1:065.

After a drumming time of 23 hours, the pelts are completely penetrated.The pre-tanned leather is washed at 30 C. for 10 minutes andsubsequently rinsed for 10 minutes.

After draining the rinsing liquor, drumming is continued with 10% of thepowdered precursory tanning agent described below. After a treatmenttime of about 4 hours the pelts are evenly tanned through.

As precursory tanning agent there is used a spray-dried condensationproduct present in the form of the ammonium salt, obtained from 1 mol(i-naphthalenesulphonic acid and 0.7 mol formaldehyde, and adjusted toan acid number of 110 by the addition of glutaric or oxalic acid.

The leather is then finally tanned with 16% pure tannin of quebrachoextract, 6% pure tannin of mimosa extract and 3% pure tannin of chestnutwood extract, all in powdered form. To avoid chafe marks, it isexpedient to add about 0.5% of a sulphited castor oil together with 1%water simultaneously with the tannin. Depending on the type of rawmaterial employed, the leather is completely tanned after a totaltanning time of 24-30 hours. The leather is then finished in the usualmanner. A smooth firm sole leather is obtained, which exhibits anegligible drawn grain or none at all.

Example 5 An unsplit cow butt pelt which has been limed in the beamhouse in the manner customary for sole leather is divided into pieces of8 by 30 cm. The pieces are treated overnight in the drum in 300% water,2% ammonium sulphate, 1.5% sodium metabisulphate and 0.5% of formicacid. After discharging the liquor (pH value about 6), one part of thecompletely delimed pelt pieces the cross-section of which is coloured ina level yellow to yellow-green shade according to the indicatorbromothymol blue, are pickled with water and 4.8% of a technicalnaphthalene-sulphonation mixture mainly containing,B-naphthalene-sulphonic acid (acid number 370).

The other part of the comparable pelt pieces are pickled with 100%water, 3.8% sodium bisulphate and 7% sodium B-naphthalene-sulphonate.

The pickled pieces are then individually pre-tanned without a liquor, ina drumming machine as has been described by E, Komarek and G. Mauthe inDas Leder 12 (1961), pages 285-289, particularly page 287, with 2.25% ofa powdered 33% basic chromic sulphate (chromium oxide content 25 untilthe whole cross-section of the pelt is tanned through. The pre-tannedleather is then rinsed with water at 35 C. and the pieces are finallytanned, again individually, in the drumming machine without a liquorwith 30% of a powdered sulphited quebracho extract.

To test the rate of tanning agent penetration, the tanning in thedrumming machine is interrupted and the leather is cut. The pelt pieceswhich have been pickled with the sodium salt of naphthalene-sulphonicacid are tanned through more rapidly than the control samples pickledwith naphthalenesulphonic acid. A comparison of the rate of tanningagent penetration with that of pelts which have been pickled withsulphuric acid and sodium chloride only, also shows that the rate oftanning agent penetration is noticeably faster in the case of the peltspickled with sodium-naphthalene-sulphonate and sodium bisulphate.

Example 6 Bellies suitable for the preparation of case leather aresoaked and limed in the customary manner. After short rinsing with waterthe pelts are delimed and bated in the drum with 200% (referred to theweight of the pelts) of water of 25 to 30 C., 1% of ammonium sulphate,0.2% of soduim metabisulphite and 0.5 of a customary bating agent. Thepelts are then rinsed and pickled with 100% of water, 1.2% of sulphuricacid (66 B.), 3% of fl-naphthalene sulphonic acid in form of its sodiumsalt, 3% of sodium sulphate or potassium sulphate and 0.5 of 85% formicacid. After a drumming time of 2 hours the pelts are evenly acidifiedand exhibit a pH of 3.5 through the entire cross-section.

The pelts are then pretanned in the pickle liquor with 10% of a powderedmixture of 80 parts by weight of 33% basic chromium sulphate (Cr Ocontent 25%) and 20 parts by weight of a delimed purified sulphite wasteliquor in form of its sodium salt. After a tanning time of 30 minutesthe liquor is buffered by the addition of sodium sulphite and sodiumcarbonate so that at the end of the chrome tannage the liquor exhibits apH of 3.8.

The evenly tanned leather is rinsed with water, neutralised and drummedwithout liquor with 6% of a powdered precursory tanning agent. Theprecursory tanning agent consists of 50 parts by weight of a neutralisedcon- 'densation product of a technical naphthalene sulphonic acid whichessentially contains fi-naphthalene sulphonic acid, and formaldehyde(molar ratio 1:065), in form of its sodium salt, and 50 parts by weightof a synthetic exchange tanning agent prepared by condensation of f3-naphthalene sulphonic acid and 4,4-dihydroxy-diphenylsulphone.

After a drumming time of about 2 hours 7% of a pure tannin of asulphited quebracho extract in powdered form and 7% of a pure tannin ofa mimosa extract in powdered form are added. The final tannage isfinished after about 4 hours. The leathers obtained are sammed, ret outand finished in the customary manner. Smooth leathers are obtained whichexhibit no drawn grain at all or only negligible drawn grain.

Example 7 In Example 6 the chromium pretanning agent is replaced by of apowdered mixture of 60 parts by weight of a 33% basic chromium sulphatetannin (Cr O content 25%) and 40 parts by weight of fl-naphthalen'esulphonic acid (Na-salt). As precursory agent a mixture of 25 parts byweight of a condensation product of ,B-naphthalene sulphonic acid andformaldehyde (molar ratio 1:1), in form of its sodium salt andspray-dried, 25 parts by weight of a delimed sulphite waste liquor, inform of its sodium salt, and 50 parts by weight of a synthetic exchangetanning agent is employed.

As exchange tanning agent the reaction product of 1 mol of phenolsulphonic acid with 1.2 mol of urea and 1.7 mol of formaldehyde,after-treated with 0.75 mol of phenol and 0.56 mol of formaldehyde,neutralised with ammonia, spray-dried and adjusted to an acid number of120 by means of glutaric acid is used.

If otherwise proceeded as described in Example 6 smooth leathers areobtained which exhibit no drawn grain at all and which show a full andround handle.

Example 8 Pelt butts suitable for the preparation of sole leather splitsare soaked and limed in the customary manner. The pelts are then drummedwith about 200% (referred to the weight of the pelt) of water of 28 to30 C. and 0.2% of hydrochloric acid for 10 minutes. 1.5% of ammoniumchloride and 1% of sodium metabisulphite are then added. After adrumming time of 1.5 to 2 hours the pelts are evenly acidified throughthe entire cross-section. If necessary, 0.2 to 0.3% of hydrochloric acidor formic acid are further added. The pelts shall exhibit a pH of 5.5 to6 through the cross-section.

The liquor is completely removed and after short rinsing 100% of water,1.2% of sulphuric acid, 3% of the ammonium salt of B-naphthalenesulphonic acid and 3% of potassium chloride are added. After furtherdrumming for 2 hours 4% of a chromium-containing pretanning agent inpowdered form are added. This pretanning agent consists of equal partsby weight of a 33% basic chrominum sulphate tanning agent (Cr O content25%) and a spray-dried condensation product of 1 mol of phenol sulphonicacid and 1 mol of urea and 1.55 mol of formaldehyde, in form of itssodium salt. After a tanning time of 3 hours the pelts are evenly tannedthrough,

The pretanned leathers are washed at 30 C. for 10 minutes andsubsequently rinsed with water of 30 C. for 10 minutes. The rinsingliquor is discharged and 10% of a powdered precursory tanning agent asdescribed below added and the drumming continued. As precursory tanningagent a spray-dried sodium salt of a neutral condensation product of 1mol of [Si-naphthalene sulphonic acid, 0.5 mol of o-chlorophenol and0.85 mol of formaldehyde which is adjusted to an acid number of 110 bymeans of glutaric acid is employed. The preparation of a precursory 10tanning agent of this kind is described in US. Patent No. 2,829,122.

After a tanning time of 4 hours 26% of a pure tannin of sulphitedquebracho extract in powered form are added without further addition ofliquor. The final tannage is complete after a total tannage of 24 to 30hours. The leathers obtained are finished in the customary manner andrepresent valuable sole leathers.

We claim:

1. In a process for the rapid tanning of medium-weight and heavyleather, comprising the steps of treating delimed or non-delimed peltsby:

(1) acidification;

(2) pretanning with a material selected from the group consisting of (a)inorganic chromium tanning agents; and (b) complexes or mixtures ofinorganic chromium tanning agents with at least one material selectedfrom the group consisting of syntans and sulphite waste liquor; and (3)tanning with a material selected from the group consisting of vegetabletannins, syntans, and mixtures thereof; the improvment comprisingpreparing an acidifying agent by mixing:

(A) mineral acid or salts of mineral acids giving an acid reaction inaqueous solution;

(B) salts of naphthalene sulfonic acids; and

(C) salts or the free acid form of the lower alkyl monopoly-, orhydroxy-carboxylic acid, sulfosalicylic acid or sulfophthalic acid; atleast one of (A), (B), and (C) being an ammonium salt; and by conductingsaid acidification step (1) therewith; said acidifying agent being usedin such amount to produce a pH below about 6.5 throughout thecrosssection of said pelts.

2. The process of claim 1 in which said acidifying agent comprises, inaddition, a salt of sulfurous acid.

3. The process of claim 1 in which the acidification is conducted toproduce a pH of 3.0 to 3.6 in said pelts.

4. The process of claim 1 in which said acidifying agent is a mixture ofmineral acid and salts of naphthalene sulfonic acids, said salts ofnaphthalene sulfonic acids selected from the group consisting ofammonium salts and mixtures of ammonium and alkali metal salts.

5. The process of claim 1 in which said acidifying agent contains, inaddition, (D) a neutral salt different from (A). or

6. The process of claim 1 in which the acidification is conductedwithout substantial amounts of water.

7. The process of claim 1 in which said steps of (2) pretanning and (3)tanning are conducted without substantial amounts of water.

8. The process of claim 7 in which step (1) acidification is alsoconducted without substantial amounts of water.

References Cited Otto: Das Leder, November 1963, pages 205-209, TS 940S.

Progress in Leather Science, 1920-1945, pages 181 and -197, Pub. byBrit. Lea. Mfg. Res. Assoc, London, Eng. TS 965 B 75.

GEORGE F. LESMES, Primary Examiner D. LEVY, Assistant Examiner US. Cl.X.R.

